Oh yes you did extensive tests JDW. You da man!
As Eric Rasmussen's site alludes, System 3.2 was considered at the time the max system for stock 512K macs. The mere fact that Apple began distributing only on 800K disks commencing with System 4.0, suggests that they expected anyone upgrading to that system on a 512K Mac to have already installed the 128K ROM and 800K drive upgrades (and therefore not need the HD20 INIT). Indeed to support the 512K and 512Ke on subsequent AppleTalk releases, Apple offered special Systems 3.3 and 3.4 with Finder 5.4 & 5.5, and 6.1 respectively, simultaneously with the release of Systems 4.0 & 4.1, and 6.1 – long after the System software was no longer supported on the stock 64K 512K. So I am not surprised the HD20 INIT was not part of the 4.0 installer. If you were going to install it on a stock 512K, you would have to first copy it to a 400K drive and copy the HD20 INIT to it in order to start up from it and access your HD20 to upgrade it, so Apple assumed you already had it.
Also, there were two versions of the HD20 INIT. v.1.0 is not fully compatible with System 3.2/Finder 5.3 and up, which coincided with HD20 INIT v.1.1. Not sure which version gyounk offered on his site or is testing with. I am more impressed that you are getting gyounk to respond to you. I have rarely gotten much substantiative responses from him, and he never clarified how he was able to get a stock 512K to run 6.0.8, per this link http://www.mac512.com/macwebpages/qamac128.htm despite the failure of JDW's well documented steps to do it - so not sure how reliable his tests for you are either.
But JDW definitely confirmed that the HD20 INIT is fully compatible with System 4.1 and in my experience is simply ignored if incompatible with a particular system.
As Eric Rasmussen's site alludes, System 3.2 was considered at the time the max system for stock 512K macs. The mere fact that Apple began distributing only on 800K disks commencing with System 4.0, suggests that they expected anyone upgrading to that system on a 512K Mac to have already installed the 128K ROM and 800K drive upgrades (and therefore not need the HD20 INIT). Indeed to support the 512K and 512Ke on subsequent AppleTalk releases, Apple offered special Systems 3.3 and 3.4 with Finder 5.4 & 5.5, and 6.1 respectively, simultaneously with the release of Systems 4.0 & 4.1, and 6.1 – long after the System software was no longer supported on the stock 64K 512K. So I am not surprised the HD20 INIT was not part of the 4.0 installer. If you were going to install it on a stock 512K, you would have to first copy it to a 400K drive and copy the HD20 INIT to it in order to start up from it and access your HD20 to upgrade it, so Apple assumed you already had it.
Also, there were two versions of the HD20 INIT. v.1.0 is not fully compatible with System 3.2/Finder 5.3 and up, which coincided with HD20 INIT v.1.1. Not sure which version gyounk offered on his site or is testing with. I am more impressed that you are getting gyounk to respond to you. I have rarely gotten much substantiative responses from him, and he never clarified how he was able to get a stock 512K to run 6.0.8, per this link http://www.mac512.com/macwebpages/qamac128.htm despite the failure of JDW's well documented steps to do it - so not sure how reliable his tests for you are either.
But JDW definitely confirmed that the HD20 INIT is fully compatible with System 4.1 and in my experience is simply ignored if incompatible with a particular system.