It’s been a couple of years since I first wrote a little about the ‘H’ reference number. This is the reference you sometimes see under the bottom bracket of an SBDU frame. Well since then, I’ve dug into the SBDU data, written a couple of timelines and become a bit more clued up on that ‘H’ letter.
I say ‘clued up‘ but what I actually mean is that I’ve looked at some columns of data – understanding that data and figuring out how the ‘H’ matches up against the SB frame number it sits next too is another thing! What I know for sure, based on my small sample of data* is that the ‘H’ appears on frames towards the end of 1975 to the later part of 1979. That is a small amount of time compared to how long Ilkeston existed. So what was going on? What about 1974/75 and all the years from 1980 onward?
The ‘H’ was meant to denote some kind of deviation away from a known standard; that is the commonly held belief. But unless you had knowledge of the build sheet or you were the original purchaser of the frame, the significance of that ‘H’ was lost. It may have been more frame clearance, a different gauge of tube, a longer or shorter top tube or any kind of change in geometry or frame detail.
It was fair to assume during the period that the ‘H’ appeared on frames that a frame with an ‘H’ number was ‘different’ somehow and a frame without an ‘H’ was stock or built to known standards. But as I said above, the ‘H’ was only used for a short period so something must have changed in production towards the end of 1975 when the ‘H’ began and something must have happened late 1979 for it disappear from frames.
The very first ‘H’ ref I’ve seen is H14, and that is in the low SB400s. SB534 has H55 but SB576, a later frame, has H31, so the first thing to note about this reference system is it isn’t applied to a frame in a sequential manner in the same way as the SB number. My opinion of this is that orders could have been placed, e.g. the H31 order was placed but for some reason, the build of that frame was held, or took longer. H55 may have been placed after H31 but that frame was built before SB576. This anomaly isn’t a one off, it occurs all the way through the SB number system. SB2434 has H549 but a frame built slightly later, SB2445 has H465. There are no records or any paperwork from Ilkeston so this administrative process of allocating a reference number, and order to build timescales, may well remain a mystery.
Both SB534 and SB576 are very early examples of Reynolds 753 frames. These date to the beginning of 1976. Both of these frames are also listed as Time Trial Specials. Reynolds 753 was introduced to the public at the Paris Cycle Show in October 1975.
Therefore, my opinion is that the ‘H’ reference seems to have been introduced at the end of 1975 and it corresponds with the introduction of Reynolds 753 and the start of orders for 753 frames and specials being received by the SBDU.
That brings me to why it all seemed to disappear in 1979, and I think that all comes down to the amount of frames being produced. I have several frames in the dataset from mid to late 1979 and early 1980 and none of these have an associated ‘H’ reference stamped on the frame, so I’m confident, based on that data, that late 1979 was the point at which the ‘H’ stamp disappeared.
When you look at the data and especially at the percentage of ‘H’ referenced frames compared to standard SB numbered frames in the same period, approximately 33% of 1976 frames had an ‘H’ ref. So maybe 3 out of every 10 frames had the ‘H’.
But by the end of 1978 and the start of 1979, the percentage of ‘H’ reference frames compared to standard SB numbered frames in the same period seemed to increase massively. From my research, 1979 started at approx SB2600; my own frame, SB2692 has ‘H’ ref 591, but the last frame I’ve seen with an ‘H’ ref is SB3177 and has H854. (The highest H ref I have seen stamped is H879 on SB3143). Just looking at simple figures, from SB2692 to SB3177, you have a difference of 485 SB frames and 263 ‘H’ references (591 – 854). Even when you factor in the variations of how the ‘H’ ref correlated to the SB number that I mentioned earlier, it is still a high percentage; every other frame being built was allocated an H ref.
With that amount of growth it would probably not be long until every SB frame would need an ‘H’ reference. In my opinion the growing percentage of ‘H’ frames confirms the amount of choice and options that the SBDU were offering. It appears that the ‘H’ reference was introduced in late 1975 to identify and track custom orders but it was on every other frame by 1979. As the H ref system was approaching the 1000 point, I believe that the H stamp disappeared from the frames but continued on build sheets.
When you see an ‘H’ reference it immediately tells you that something may be different about the frame, not in a bad way, just different. But from the end of 1979, you may never know if a frame is stock or custom and you need to check geometry and especially things such as top tube length, wheelbase, brake drop etc. I’ve bought a couple of frames lately that have shorter than normal top tubes which is ok for me as that is what I need.
When you are buying a 2nd hand SBDU frame you are buying a frame built for someone else, built to their body size, built to their spec of components and built to their racing purpose. You don’t want to invest in one of these frames based exclusively on something like seat tube length if ultimately, the frame won’t fit you or your intended use.
As always, as more frames and more data becomes available, the assumptions I’ve made may change and require updating.